Advances in polymer technologies have enabled the widespread use of relatively lightweight and lower cost molded plastic components in applications that formerly required the use of metal components. In many applications plastic components are secured to other components using compressive fasteners such as bolts or screws. It is known that applying excessive compressive forces to plastic components using such fasteners can lead to cracking or mechanical failure of the plastic component. It is therefore desirable to provide a compression limiter configured to limit compressive forces applied to the plastic component by conducting excessive compressive force away from and around the plastic component.
To that end, various types of compression limiters have been developed and applied in the art. For example U.S. Pat. No. 5,513,603 provides a fastener isolation system for mounting a thermoplastic valve cover on an engine. The fastener includes an elastic bushing or washer and a relatively incompressible metallic sleeve limiting compression that may be applied to the thermoplastic valve cover by the fastener.
U.S. Pat. No. 5,807,052 discloses another variety of compression limiter having a sleeve member extending through an opening provided between the outer and inner surfaces of a work piece. The sleeve member includes a substantially radial flange member configured to engage a surface of the work piece.
It is further known than molded plastic materials, when subject to compressive forces over time, are susceptible to a permanent material shape distortion known as “creep”. When a plastic component is initially fastened using a fastener such as a bolt, the bolt applies an initial compressive load to the plastic component. Over time the plastic material deforms or “creeps” in a manner to reduce the applied compressive load of the bolt at the point of contact. As the compressive load decreases, the component is no longer securely held in place and may be free to shift or to vibrate, defeating what may have been a robust component mounting.
Unfortunately, the past methods and apparatus for limiting compression in mounting plastic components, while quite functional for mounting individual plastic components to a substrate, do not offer a solution for securing together stacks of plastic or other creep-susceptible components. As plastic materials are known to change dimensions or creep over time, the compressive fasteners such as bolts applied to secure a stacked assembly of plastic components together may loosen over time.